[tharbert]

I vote #1 - BTDT

Fit each tread and complete final sand
stain and finish - I prefer hard wearing satin polyurethane...
Cut your filler with some of your stain...goal is to get it close to the right color
--You can also get colored filler too
Predrill for your nails
Hammer nails down to the head and use punch to sink below flush
Apply filler and stain to match - I used a Q-tip
Lightly sand flush with ~400 grit
Put on final full coat of polyurethane

Personally, I would use a hardword. White pine is, IMHO, too soft for treads. Veneers may not hold up to the daily pounding. +1 hard maple or oak. I like alder.

BTW, in the staircase picture, the finished wood looks like maple...